Loads of various types can be driven by a pulsed current source, in which the width, or duty ratio, of pulses controls the amount of current supplied to the load. An example is in a circuit for driving a light emitting diode (LED) with a pulsed current source, in which the pulse width is varied in order to control light intensity produced by the LED. Pulsed current is generated from an unregulated input voltage supply by enabling and disabling a voltage regulator to drive the desired current through the LED. If the pulse is mainly on (high duty ratio), LED light intensity is high. As duty ratio is lowered, the LED will appear to dim.
A variety of regulator topologies has been implemented to generate pulsed current of varying duty ratio, namely Buck, Boost, and Buck-Boost converters, each of which employs an inductor and filter capacitor to generate regulated voltage. FIG. 1 shows a switching regulator 10 in the Buck configuration driving an LED load 12, which may comprise a single LED or a network of LEDs in series, parallel or series-parallel configuration. A series pair of LEDs is depicted in FIG. 1 by way of example. Converter 10 comprises a bipolar transistor 12, Schottky diode 14, inductor 22, capacitor 23 and resistor 16, connected as shown. Connected across resistor 16 through which a current ILED flows to activate the LED load 12, is an amplifier 18. The inverting input of amplifier 18 is connected to unregulated input voltage source VIN at node 14 through a fixed reference voltage source 20 of magnitude VREF and of polarity shown. Amplifier 18 accordingly generates a signal that indicates when the voltage drop across current sense resistor 16 exceeds the level of reference voltage VREF source 20. Coupled to the output of amplifier 18 is an on/off modulator 17, cycled by an input pulse width modulation (PWM) signal applied at node 24.
As the operation of a Buck type switching regulator is well known, the same will not be described herein, for brevity.
The conventional regulator 10 of type depicted in FIG. 1 has inherent deficiencies, making it poorly suited to precision applications such as in PWM dimming of driven LEDs. For example, the FIG. 1 switching regulator tends to maintain voltage across the LED load after the regulator turns off. An unregulated current accordingly continues to flow through LED 12 for some time period after turn-off, until capacitor 23 discharges. In addition, restoration of load current is delayed by the amount of time it takes to recharge capacitor 23 once the regulator is re-enabled. This characteristic is unacceptable in precision LED dimming circuits, where accurate control of LED intensity may be important to maintain constant LED color over a wide range of brightness. As DC current through an LED changes, so does the color of light the LED emits. For example, a blue LED will still be generally blue whether it is driven at 100 mA or 1 mA, but the wavelength of light emitted from it will change significantly. It is important to applications in which red/green/blue LEDs are mixed, for example, to achieve a desired white light, to accurately control LED emission.
To confront capacitor charging and discharging delay in the conventional regulator, PWM dimming has been improved by introducing a switch in series with the LED load, to interrupt current flow through the LED during times when the regulator is turned off. This technique employs either an NMOS (N-channel metal oxide semiconductor field effect transistor) switch to disconnect the load from the low voltage side (e.g., ground side) of the output voltage (termed low side LED dimming) or a PMOS (P-channel metal oxide semiconductor field effect transistor) switch on the high side (e.g., power side) to disconnect the LED load from the high voltage side of the output voltage (high side LED dimming). Either technique interrupts the discharge path of the regulator capacitor.
In the example of a low side LED dimmer circuit, the NMOS switch can be driven with the same signal as the PWM signal that enables the regulator. However, a high side LED dimmer implements a PMOS switch that must be driven with an inverted version of the PWM signal that is level shifted to the PMOS source voltage.
The low side dimming approach can be employed to extend PWM dimming ratio for the Buck converter shown in FIG. 1. A low side dimming circuit 30 and part of the Buck converter are shown in FIG. 2. The PWM dimming ratio is improved by connecting a NMOS transistor 34 in series with LED load 32. The gate of transistor 34 is switched by the PWM signal applied to an NMOS transistor 36, connected between transistor 34 and ground. A resistor divider consisting of resistors 40 and 42, connected as shown, establishes operating voltage levels for the circuit.
Although an improvement over the conventional circuit, the low side PWM dimming circuit of FIG. 2 has shortcomings. The values of resistors 40 and 42 are difficult to establish, gas magnitudes of VIN and VOUT with respect to ground must be known precisely for establishing the appropriate resistance values needed in order to turn off NMOS switch 34 when the PWM signal is high. When PWM is low, NMOS transistor switch 36 turns off, and switch 34 is turned on by resistor 40. If VIN-VOUT is too high, an additional circuit must be recruited to prevent over-voltage from destroying the gate of the NMOS switch 34. For this reason, a high side PWM dimmer circuit of the type shown in FIG. 3 often is preferred.
Referring to FIG. 3, high-side LED dimmer circuit 40 implements an NMOS transistor 46 to ground and resistor divider 42,54 to perform inversion and voltage level shifting to enable PMOS switch 44 to be cycled in response to a PWM signal applied at node 48. The resistor divider 42,54 is necessary to limit the voltage between source and gate electrodes of PMOS switch 44, required when source voltage exceeds 20 volts above ground. This method of driving the PMOS gate, however, has several disadvantages. Resistor divider 42,54 imposes an RC time constant on the switch turn-on and turn-off times. The resistor divider ratio generally is fixed, with the undesirable result that the gate to source voltage applied to switch 44 is proportional to the supply voltage at VIN, which is unregulated and, hence, variable. In addition, the resistor divider 42,54 draws substantial current from the power supply during the time when the PMOS transistor 44 is turned on. The present disclosure describes an improved circuit that addresses the aforementioned deficiencies.